The organization of regulatory DNA elements into precise chromatin structures is important for both DNA replication and transcription in vivo (Lee et al. (1993) Cell 72:73-84; Felsenfeld (1992) Nature. 355:209). In eukaryotic cells, nuclear DNA exists as a hierarchy of chromatin structures, resulting in the compaction of nuclear DNA about 10,000 fold (Davie and Hendzel (1994) J. Cell. Biochem. 55:98). The repeating structural unit in the extended 10 nm fibre form of chromatin is the nucleosome (van Holde (1988) Chromatin. New York: Springer-Verlag). The nucleosome consists of 146 by of DNA wrapped around a protein core of the histones H2A, H2B, H3, and H4, known as the core histones. These histones are arranged as an (H3-H4)2 tetramer and two H2A-H2B dimmers positioned on each face of the tetramer. The DNA joining the nucleosomes is called link DNA; it is to the linker DNA to which the H1 or linker histones bind. The 10 nm fibre is compacted further into the 30 nm fibre. Linker histones and amino-terminal regions (“tails”) of the core histones maintain the higher order folding of chromatin (Garcia Ramirez et al. (1992) J. Biol Chem 267:19587). This chromatin structure must be relaxed when DNA is transcribed or translated.
Histones of the nucleosome core particle are subject to reversible acetylation at the ε-amino group of lysines present in their amino terminus (Csordas et al. (1990) Biochem J 265:23-38). Transcriptionally silent regions of the genome are enriched in underacetylated histone H4 (Turner (1993) Cell 75:5-8), and histone hyperacetylation facilitates the ability of transcription factor TFIIIA to bind to chromatin templates (Lee et al. (1993) Cell 72:73-84). Recent genetic, biochemical and immunological approaches have provided substantial evidence indicating that histones associated with actively transcribed genes are more highly acetylated than those from nontranscribed regions. While not wishing to be bound by any particular theory, histone acetylation may influence transcription at several stages, for example, by causing transcription factors to bind or by inducing structural transitions in chromatin, or by facilitating histone displacement and repositioning during polymerase elongation.
The acetylation and deacetylation are catalyzed by specific enzymes, histone acetyltransferase and deacetylase, repectively, and the net level of the acetylation is controlled by the equilibrium between these enzymes. The steady state level of acetylation and the rates at which acetate groups are turned over vary both between and within different cell types, with half-lives that vary from a few minutes to several hours. Although a histone acetyltransferase gene (HAT1) has been identified in yeast (Kelff et al. (1995) J. Biol. Chem. 270:24674-24677), the molecular entities responsible for histone deacetylation were heretofore unknown in the art.
The identification of the mechanism by which histones are deacetylated would be of great benefit in the control of gene transcription and the cell cycle.